This invention relates to improvements in an ultrasonic measurement method and apparatus for subjecting an object to an ultrasonic transmission and receiving reflected ultrasonic waves from the interior of the object to measure the acoustic characteristics of the object. More particularly, the invention relates to an ultrasonic measurement method and apparatus for measuring, by approximation, attenuation coefficient and reflection coefficient ascribable to propagation of ultrasonic waves internally of an object, thus to obtain information relating to the attenuation and reflection of the ultrasonic waves within the object.
Ultrasonic measurement techniques find application widely in such fields as material testing, SONAR and medical diagnosis. In particular, ultrasound scanner systems for medical purposes have recently been developed.
The principle of operation of an ultrasound scanner apparatus resides in use of a pulse-echo method and utilizes a phenomenon wherein an ultrasonic pulse transmitted into a living body, which is the object undergoing measurement, is reflected at a boundary where there is a difference in acoustic impedence. The reflected wave (echo) is received and processed to display a tomograph of the living body by a so-called B-mode method. Despite the fact that the echo contains a variety of information such as the ultrasonic attenuation, acoustic impedence and propagation velocity of sound, the information utilized at the present time is solely the amplitude of the echo.
More specifically, the propagation velocity of sound in the biological tissue is assumed to be constant and, with regard to attenuation ascribable to ultrasonic propagation, luminance modulation is performed at the value of the echo amplitude arbitrarily corrected by a so-called STC (sensitivity time control) circuit or TGC (time gain control) circuit, with the modulated signal being displayed as a tomograph on a cathode-ray tube. This is referred to as a "B-mode display". Accordingly, the tomograph obtained is nothing more than a qualitative picture of a two-dimensional distribution at a surface where the acoustic impedence is discontinuous, so that the morphological information relating to the position and shape of the biological tissue inevitably forms the core of the information utilized. However, the state of the art is such that information such as that relating to ultrasonic attenuation, which is a characteristic of the biological tissue, is not measured.
Several attempts at attaining attenuation information relating to biological tissue have been reported. However, as will be described below in further detail, an echo waveform contains two types of information, namely attenuation due to propagation through biological tissue, and coefficient of reflection at an interface or boundary where there is a difference in acoustic impedence. Both of these quantities are unknown. Therefore, distinguishing between the effects of these two quantities and recognizing them is extremely difficult at the present time.
If the reflected intensity is assumed to be independent of the frequency of the ultrasonic waves and ultrasonic waves having two or more frequencies are transmitted and the ultrasonic echo received with regard to the same portion of the object under measurement followed by measuring the sound pressure ratio of each frequency component of the echo, then it will be possible to eliminate the influence of the reflected intensity and derive an attenuation coefficient. The foregoing assumption holds in the case of an acoustic interface having a sufficiently wide spread in comparison with the wavelength of the ultrasonic waves, e.g. in the case of a planar reflector. In actuality, however, a scatterer approximately equivalent to or less than the wavelengths used often resides at the biological tissue. It is therefore difficult to consider that the foregoing assumption will always hold for the entirety of a biological tissue.
In addition, if it is assumed that the reflected intensity is approximately constant at a certain portion of a biological tissue, then one may consider that the echo sound pressure ratio across the front and back of this portion of the tissue is proportional to the attenuation coefficient. Further, experiments have been reported wherein an attenuation coefficient is obtained by presupposing a relation giving the frequency dependence of the reflected intensity, transmitting ultrasonic waves having three or more frequencies, receiving the ultrasonic echo with regard to the same portion of the object under measurement, and measuring the sound pressure of each frequency component of the echo, with the attenuation coefficient being obtained from the sound pressure.
Thus, the method employed to isolate and measure an attenuation coefficient in all of the foregoing cases involves making an assumption with regard to the reflected intensity, as well as transmitting and receiving ultrasonic waves having a single frequency component or a plurality of frequency components.
A well-known method of measuring attenuation coefficient relies upon transmission. Specifically, as shown in FIG. 1, a transmitting probe 1 and a receiving probe 2 are disposed so as to confront each other across a specimen 15. If ultrasonic waves are transmitted and received at a frequency f, then the following equation may be obtained giving the relation among the amplitude V.sub.o (f) of the transmitted waves, the amplitude V.sub.r (f) of the received waves, and the attenuation coefficient .alpha.(f,x): ##EQU1## where the attenuation coefficient .alpha.(f,x) contains an attenuation coefficient and a forward-scatter coefficient (transmittance). L denotes the total length of the path across the specimen. Taking the natural logarithm of both sides of this equation and transforming gives the following: ##EQU2## Next, by using an X-ray computerized tomographic technique, the projection data may be collected to obtain .alpha.(f,x) through use of a well-known filtered back-projection algorithm, by way of example.
However, a problem is encountered in obtaining .alpha.(f,x) in this fashion. Specifically, as shown in FIG. 2 by way of example, if a scatterer 3 which exhibits a comparatively high degree of scattering is located at a portion of the specimen 15, there is an apparent decline in the reception amplitude V.sub.r (f) which arrives at the receiving probe 2 (see the graphical representation on the right-hand side of FIG. 2). As a result, the reconstructed picture of the attenuation coefficient distribution displayed on the display unit will give one the impression that a large attenuation coefficient exists at the boundary of the scatterer 3. In other words, the attenuation coefficient, rather than being a pure attenuation coefficient, is instead clearly influenced by the scattering intensity of the scatterer.